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March 21 (Reuters) - U.S. power company Entergy Corp installed a temporary cover over an opening in the reactor building at the Vermont Yankee nuclear power plant after a so-called "blowout" panel opened on Monday, nuclear regulators said Thursday.
The issue did not cause any danger to the public or plant workers, the U.S. Nuclear Regulatory Commission (NRC) and Entergy told Reuters.
The reactor is currently shut for planned refueling and maintenance. During testing of a reactor building ventilation system on Monday, a roughly 6-by-10 foot (1.8-by-3 meter) "blowout" panel blew out due to an over pressurization condition.
Entergy spokesman Jim Sinclair said the panel blew out as designed when pressure slightly increased. "It has been replaced by a panel that meets/exceeds the original design," he said.
The panel opening had no impact on the refueling outage and was not reportable under NRC regulations as it was an event of no consequence, Sinclair said.
He could not say when the unit was expected to return to service. Energy traders guess the unit, which shut for refueling on March 9, will return in mid April.
The NRC said the blowout panels are located on the refueling floor and are there to protect against tornado-force winds that can cause over pressurization conditions in the reactor building.
If the reactor building is threatened by tornado-force winds, the panels can blow out and provide near instantaneous pressure equalization in the building. Other buildings, including the turbine building, are also equipped with blowout panels, the NRC said.
Separately, the NRC on Wednesday sent a letter to Entergy requesting additional financial information following a Securities and Exchange (SEC) filing by the company in November about a financial impairment of Vermont Yankee.
The NRC staff said it wanted the additional information to ensure that Entergy is meeting NRC requirement for financial qualifications.
In the SEC filing, Entergy said it has "tested the recoverability of the plant and related assets each quarter since the first quarter 2010 ... because of the uncertainty regarding the continued operation of Vermont Yankee."
That was about the start of Entergy's battle with some Vermont politicians to keep the reactor operating for an additional 20 years as the company sought a new federal operating license. The NRC renewed the plant's operating license in March 2011.
That battle between the state and the company is ongoing in federal and state court appeals and regulatory proceedings at the state Public Service Board.
Prior to the first quarter 2012, the company said "the probability-weighted undiscounted net cash flows exceeded the carrying value of the Vermont Yankee plant."
But, the company said the decline "in the overall energy market and the projected forward prices of power as of March 31, 2012 ... resulted in the probability-weighted undiscounted future cash flows being less than the asset group's carrying value."
Power prices in 2012 fell to at least decade lows in New England and other parts of the United States, due primarily to weak natural gas prices from record shale production.
Entergy said it estimated the fair value of the plant on March 31, 2012 was $162.0 million, while the carrying value was $517.5 million.
Therefore, the company said it wrote down Vermont Yankee to the fair value and recognized an impairment charge of $355.5 million, or $223.5 million after tax.
Sinclair at Entergy said, "We will be responding to the NRC request for information."